2. THE PERFORMANCE OF MOND

Figure 1 summarizes the mass discrepancy in
various galactic systems. It shows, approximately, the
ratio of the dynamical mass, as determine with standard
dynamics, to the mass so far accounted for by direct observations.
The discrepancy is plotted against some "typical" system radius.
(Masses in galactic systems show no sign of saturation
with radius, and the value within that "typical" radius is used.)
I note, in passing, that there is no correlation of
the discrepancy with system size.
Remark, in particular, that the small dwarf spheroidals and LSB discs
show large discrepancies, while the large galaxy clusters evince only
moderate discrepancies. This flies in the face of attempts
to explain away the mass discrepancy by modifying gravity at
large distances, predicting
increase in the "discrepancy" with size. (Contrary to some lingering
misconception, MOND is not a modification at large distances, but at low
accelerations-which for a given mass are attained at large distances.)

Figure 1. The mass discrepancy (dynamical
mass over detected mass) in various
galactic systems plotted against the typical system size.

The use of MOND dynamics should eliminate the mass discrepancy
in all systems. Put differently, MOND predicts the mass discrepancy
expected when using Newtonian dynamics.
Figure 2 shows the
discrepancy plotted now against the typical inverse
acceleration-as prescribed by MOND. It also shows the MOND prediction
of the discrepancy as a
solid line interpolating the value 1 at low a-1 and the
predicted discrepancy, a0/a, at a
<< a0.
The positions of the blobs describing the different galactic systems
roughly represent detailed work on individual systems: dwarf spheroidals
[6] -
[9],
disc-galaxy rotation curves
[10] -
[13],
galaxy groups
[14],
x-ray clusters (e.g.
[15] -
[18]),
and large-scale filaments
[19].
I expand here on two types of systems.

Figure 2. The mass discrepancy plotted
against the typical system acceleration.